AU739327B2

AU739327B2 - Hologram-type polarized-light splitting element

Info

Publication number: AU739327B2
Application number: AU21858/99A
Authority: AU
Inventors: Kazuhiro Inoko
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1998-03-24
Filing date: 1999-02-03
Publication date: 2001-10-11
Anticipated expiration: 2019-02-03
Also published as: JPH11271533A; BR9908958A; CN1294688A; US6417941B1; KR20010034621A; CA2322332A1; EP1067408A4; WO1999049339A1; AU2185899A; EP1067408A1

Description

SPECIFICATION

HOLOGRAM POLARIZED LIGHT SEPARATOR TECHNICAL FIELD TO WHICH THE INVENTION PERTAINS The present invention relates to a polarizing element for taking only unidirectional plane-polarized light out of natural light being so called indefinitely polarized light.

BACKGROUND OF THE INVENTION A conventional liquid crystal projector (LC projection display) is provided with a liquid crystal display panel that necessarily uses a polarizing plate for cutting off unnecessary components from light outgoing therefrom for forming an image.

However, a conventional dichroic type polarizing plate absorbs unnecessary light that becomes heat energy for internally heating the plate, causing premature deterioration of the plate.

Accordingly, recent LC projectors use, instead of the polarizing plate, a polarized-beam splitter having a thin-film coat deposited thereon or diffraction type polarizing elements disclosed in Japanese Laid-Open Patent PublicationNo. 61- 240204 and No. 62-249107 and Japanese Patent No.2594548 (Japanese Laid-Open Patent Publication No. 63-26604).

However, the above described conventional methods have the following drawbacks: The thin-film deposited type polarized-beam splitter is constructed of two glass-made prisms bonded to each other, which is thick and, therefore, is not so small and light to be usable in liquid crystal. projectors. Namely, the use of this type beam splitter as a polarizing plate of a light outputting portion of a liquid crystal projector necessarily elongates a back focal length of a projector lens and, thereby, causes the need for increasing a diameter of the projection lens, a projection distance and a weight of a whole optical system.

The application of the diffraction type polarizing element described in Japanese Laid-Open Patent Publication No. 61- 240204 as an emergent-side polarizing plate cannot produce a well-contrasted clear image since the element cannot completely split polarized light, causing leakage of light. The diffraction type polarizing element described in Japanese Patent No.2594548 (Japanese Laid-Open Patent Publication No.

63-26604) as compared with the element of Japanese Laid-Open Patent Publication No. 61-240204 has a higher degree of polarized-beam separation, a higher contrast and a smaller wavelength-dependent diffraction difference that is estimated at a value of dispersion of refracting media. This element, however, cannot have a large diffraction angle because of a small differential refractive index of its diffraction grating.

The use of this element as a polarizing plate on the emergent side of a liquid crystal projector elongates a back focal length of a projection lens. The hologram-type polarized-light splitting element described in Japanese Laid-Open Patent P:\PDOCSmd\spai\7514380.docI4 Augw1 2001 -3- Publication No. 62-249107 has an excellent polarised-light splitting ability and a large splitting angle but has an incident light falling thereto at a large angle to a line perpendicular to the hologram plane, requiring a large working space. Therefore, the use of this element as an emergent-side polarising plate in a liquid crystal projector may also require elongation of a back focal distance of the projection lens.

::In view of the foregoing, the present invention was made to provide a hologram polarised-light splitting element capable of effectively splitting polarised light with no absorption of unnecessary light, eliminating the possibility of being heated by 10 absorbed light and of deterioration by heat, which element is so light and small to form a compact optical system. The present invention is also directed to a compact liquid crystal *display device using the above-described element.

SSUMMARY OF THE INVENTION 15 The present invention seeks to provide a hologram-type polarised-light splitting element made of a glass substrate with a hologram formed thereon, which element can split incident light into two plane-polarised (linearly polarised) components having polarisation directions perpendicular to each other by allowing one of the components to pass the hologram after diffraction and the other component to pass straight with diffraction, and which element can select and emit only the non-diffracted plane-polarised component of the light in the specified direction and, at the same time, can further reflect the diffracted plane-polarised component at least once at a boundary between the glass substrate and medium surrounding the element. This element can thus split incident light at high P:AWPDOCS\.mdXjpcc\70I4ig0,doc.I4 August 2001 -4accuracy with no absorption of unnecessary light component that may become thermal energy as observed in the conventional dichroic absorption type light polarising element, thereby it can maintain high reliability of performance for its long service life. The element is a thin plate that can form a very compact and light optical system in which it can be disposed at right angles to an optical axis of the system.

The present invention further seeks to provide a hologram-type polarised-light splitting element made of a glass substrate with a hologram formed thereon, in which the hologram is given diffraction properties for obtaining diffracted plane-polarised light and 10 non-diffracted plane-polarised light from a p-plane-polarised component and an s-planepolarised component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a separation angle of degrees with the travelling direction of the latter.

15 The present invention also seeks to provide a hologram-type polarised-light splitting element made of a glass substrate with a hologram formed thereon, in which the hologram is given diffraction properties for obtaining diffracted plane-polarised light and non-diffracted plane-polarised light from a p-plane-polarised component and an s-planepolarised component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a separation angle of 48.2 degrees with the travelling direction of the latter.

I0380.doc-14 August 2001 The present invention further seeks to provide a liquid crystal projection display having a compact optical system with a short back-focal distance of a projection lens, which comprises a liquid crystal panel and at least a first polarising element and a second polarising element on the incident side and the emergent side, respectively, of the liquid crystal panel, in which each polarising element is a hologram-type polarised-light splitting element according to the present invention, which element can split incident light into two i polarised components at high contrast with no fear of being heated by light absorption and has no need for elongating a back focal length of its projecting lens.

10 BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the following detailed description of the preferred but non-limiting embodiments thereof, described in connection with the accompanying drawings, wherein: 15 Fig. 1 is a view for explaining how incident light is split into diffracted light and transmitting light by means of a hologram-type polarised-light splitting element.

Fig. 2 is a view for explaining the principle of setting diffracting properties for a hologram-type polarised-light splitting element according to the present invention.

Fig. 3 is a schematic construction view of a hologram-type polarized-light splitting element embodying the present invention.

Fig. 4 is aviewforexplaininghowtoprepare a hologram-type polarized-light splitting element shown in Fig. 3.

Fig. 5 is a schematic construction view of another hologram-type polarized-light splitting element embodying the present invention.

Fig. 6 is a view for explaining how to prepare a hologram-type polarized-light splitting element shown in Fig. Fig. 7 is a schematic construction view of essential portions of a liquid crystal projection display according to an embodiment of the present invention.

Fig. 8 is a schematic construction view of essential portions of a liquid crystal projection display according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A hologram-type polarized-light splitting -element according to the present invention is a glass substrate with a transmitting-type hologram formed thereon for diffracting one of plane-polarized components of natural incident light vertically entering the element and allowing the other plane-polarized component perpendicular to the former component to pass therethrough without diffraction. The hologram has a specified grating angle, width, thickness and refractive index range so as to diffract polarized light at a specified angle necessary for obtaining high diffraction efficiency and totally reflecting light from a glass-to-air boundary of the glass substrate.

The preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

Fig. 1 is a view for explaining light rays diffracted and transmitted by a hologram-type polarized-light splitting element according to the present invention, which element comprises ahologram 1 and a grass substrate 2. In Fig. 1, there is shown natural incident light I, first plane-polarized light (transmitted light) E .and second plane-polarized light (diffracted light) E2' Fig. 2 is a view for explaining a diffraction angle selecting method applied to the hologram-type polarized-light splitting element according to the present invention, where d is a hologram thickness, 0 is an angle of incident light and Os is an angle of emergent light with respect to an imaginary line perpendicular to the hologram surface.

A hologram-type polarized-light splitting element according to the present invention is constructed of a glass substrate 2 with a hologram 1 formed thereon. The hologram 1 is a volume-type phase hologram whose material and thickness are selected so that it achieves possibly highest diffraction efficiency. The hologram 1 has such a preset diffraction angle that it may most accurately split natural light into two plane-polarized components whose polarization directions are perpendicular to each other. One component is first planepolarized light EI passing the hologram 1 without being diffracted and the other is second plane-polarized light E 2 diffracted by the hologram 1.

Referring to Fig. 2, the concept of setting diffraction properties of the hologram will be described below: According to the Kogelnik's coupled-wave theory, diffraction efficiency rT of the volume-type phase hologram can have the following expression on the condition there is no absorption by material and Bragg's condition is satisfied.

T= sin (1) Kd V= (2) ScosOcosO An angle 0 made by light not diffracted inside the hologram with the line perpendicular to the surface of the hologram is equal to an angle 0 of incident light. In the equation k is called a coupling coefficient. Polarized light p and polarized light s may have different coupling coefficients kp and k s respectively.

K nlcos(O- (3) Kp ICOS(- 0 (3) an K, (4) Consequently, p-plane-polarized light and s-planepolarized light have different values of diffraction efficiency.

A p-plane-polarized light diffracting hologram-type polarized-light splitting element defined in claim 2 may have diffraction efficiency Tp=1 for p-plane-polarized light and diffraction efficiency qs=0 for s-plane-polarized light.

Therefore, v=/2 and v=x are obtained according to Equation From Equations and the following condition can be derived.

1= 600 0 is equal to 0 when incident light enters the hologram perpendicularly to the top surface of the hologram. When Os is equal to 600, the diffraction efficiency can be set to 1 for s-plane-polarized light and the diffraction efficiency can be set to 0 for p-plane-polarized light. In this instance, the first plane-polarized light E, becomes s-plane-polarized light and the second plane-polarized light E 2 becomes p-planepolarized light as shown in Fig. 1.

For a s-plane-polarized light diffracting hologram-type polarized-light splitting element defined in claim 3, values v for s-plane-polarized light and p-plane-polarized light may satisfy the condition v=x and v=3x/2 respectively. Similarly to Equation the following condition is obtained: 1 0 1= 48.2° (6) Accordingly, the diffraction efficiency values 1 and 0 are obtained for s-plane-polarized light and p-plane-polarized light respectively when 0 is 0 and 0s=48.20. In this case, the first plane-polarized light E 1 becomes p-plane-polarized light and the second plane-polarized light E 2 becomes s-planepolarized light as shown in Fig. 1.

The natural light I entering the hologram 2 at right angles thereto is split into the first plane-polarized light E 1 passing the hologram without being diffracted and the second planepolarized light E 2 is diffracted at the specified angle by the hologram 2. The polarization direction of the second planepolarized light E 2 is normal to that of the first plane-polarized light. The first non-diffracted plane-polarized light El passes through the glass substrate 2. On the other hand, the second diffracted plane-polarized light E 2 travels in the glass substrate 2 and completely reflected into the glass from a glass-to-air boundary if the glass substrate 2 has a refractive index (ng) of 1.52 with a refraction angle exceeding the critical angle of 410 at the boundary. Therefore, both plane-polarized splitting elements of claims 2 and 3 can selectively output only one of two plane-polarized components of incident natural light in a specified direction.

Fig. 3 is a schematic construction view of a hologram-type polarized-light splitting element embodying the present invention. In Fig. 3, numeral 11 designates a hologram and numeral 12 designates a glass substrate.

Fig. 4 is a view for explaining how to prepare the hologram-type polarized-light splitting element shown in Fig.

3. There is shown a photosensitive material lle, a first trapezoidal prism 13a, a second trapezoidal prism 13b, objective light O and reference light R. Other elements similar to those shown in Fig. 3 are given the same reference letters and symbols.

As described above with reference to Equation the hologram can split incident plane-polarized light into two components by diffracting p-plane-polarized light at an angle of 600 to a non-diffracted s-plane-polarized light. A hologram made of photosensitive material, photo-polymer (n=1.54) having thickness of 12.2 microns will be described below.

The hologram is prepared by exposing the photosensitive material through an optical system shown in Fig. 4. The first and second trapezoidal prisms 13a and 13b have a trapezoidal section having an angle of 61.30 at one of four corners and are made of the same glass material (ng=1.52) that used for making the glass substrate 12. The trapezoidal prisms 13a and 13b are symmetrically attached with matching oil to opposite surfaces of a holographic dry plate that is a photosensitive material lie made of photo-polymer and attached to the glass substrate 12. The dry plate is exposed to reference light R and objective light O coherent with the reference light R in such a way that light rays R and O enter the oblique surface of the prism 13a, making an'angle of 63.10 with a line perpendicular to the glass substrate 12 as shown in Fig. 4. The objective light O is refracted at the boundary from the first trapezoidal prism 13a to the photosensitive material lie, making an angle of 600 with the normal of the hologram, and interferes with the reference light R to form interference fringes inside the photosensitive material lle. The trapezoidal prisms 13a and 13b are removed, then the exposed photosensitive material is processed to obtain a hologram-type polarized-light splitting element. In this instance, the prepared element is supposed to have the hologram with grating fringes recorded therein with a refractive index amplitude nl of 0.03. The spacing between grating fringes formed in the hologram is 0.33 microns when the hologram was exposed to the light of 514 nm. The normal line of the grating fringes makes an angle of 600 with the normal line of the hologram.

When natural light having awavelength of 514 nm enters the prepared hologram-type polarized-light splitting element along the normal line of the glass substrate 12, it can be split into two plane-polarized components s and p: the p-planepolarized component is diffracted by the hologram ll and totally reflected from the glass-to-air boundary, while the splane-polarized component travels straight (without being diffracted) and passes the glass substrate 12 as shown in Fig.

3. The wavelength range of the element can be expanded by overlaying a plurality of holograms or conducting multiple exposure of the photosensitive element.

Fig. 5 is a schematic construction view of another hologram-type polarized-light splitting element embodying the present invention. In Fig. 5, there is shown a hologram 21 and a glass substrate 22. Fig. 6 is a view for explaining how to prepare the hologram-type polarized-light splitting element of Fig. 5. In Fig. 6, there is shown a photosensitive material 21e, a first trapezoidal prism 23a, a second trapezoidal prism 23b, an objective light O and reference light R. Other elements similar to those of Fig. 5 are given the same reference characters.

As described before with reference to Equation the hologram can split incident light into two plane-polarized light components s and p when it may diffract the s-plane component at an angle of 48.20 with respect to the non-diffracted p-plane component. A hologram made of photosensitive material, photo-polymer (n=1.54) having thickness of 12.2 microns will be described below. The hologram is prepared by exposing the photosensitive material 21e through an optical system shown in Fig. 6. The first and second trapezoidal prisms 23a and 23b have a trapezoidal section having an angle of 490 at one of four corners and are made of the same glass material (ng=1.52) that used for making the glass substrate 22. The trapezoidal prisms 23a and 23b are symmetrically attached with matching oil to opposite surfaces of a holographic dry plate that is the photosensitive material 21e made of photo-polymer and attached to the glass substrate 22. The dry plate is exposed to reference light R and objective light O coherent with the reference light-R so that light rays R and O enter the oblique surface of-the prism 13a making an angle of 48.20 with a line perpendicular to the glass substrate 22 as shown in Fig. 6. The objective light O is refracted at the boundary from the first trapezoidal prism 23a to the photosensitive material 21e, making an angle of 65.90 with the normal of the hologram, and interferes with the reference light R to form interference fringes inside the photosensitive material 21e. The trapezoidal prisms 23a and 23b are removed, then the photosensitive material is processed to obtain a hologram-type polarized-light splitting element. In this instance, the prepared element is supposed to have a hologram with grating fringes recorded therein with a refraction factor amplitude nl of 0.03. The spacing between grating fringes formed in the hologram is 0.41 microns when the hologram was exposed to the light of 514 nm.

The normal line of the grating fringes makes an angle of 65.90 with the normal line of the hologram.

When natural light having a wavelength of 514 nm enters the prepared hologram-type polarized-light splitting element along the normal line of the glass substrate 22, it can be split into two plane-polarized components s and p: the p-planepolarized component is diffracted by the hologram 21 and completely reflected from the glass-to-air boundary, while the s-plane-polarized component travels straight (without being diffracted) and passes the glass substrate 22 as shown in Fig.

The wavelength range of the element can be expanded by overlaying a plurality of holograms or conducting multiple exposure of the photosensitive element.

Fig. 7 is a schematic construction view showing essential portions of a liquid crystal projection display according to an embodiment of the present invention. In Fig. 7, the liquid crystal display comprises a liquid crystal display panel 31, a first hologram-type polarized-light splitting element 32a and a second hologram-type polarized-light splitting element 32b.

In this embodiment, the liquid crystal display panel 31 is provided with the first and second polarized-light splitting elements 23a and 23b disposed before and after thereof. Natural light from a light source (not shown) through an optical system (not shown) enters the first hologram polarized-light splitting element 32a by which it is split into two plane-polarized components whose polarization directions are perpendicular to each other. One component is diffracted by the hologram and then totally reflected from the glass substrate, not reaching the liquid crystal display panel 31. The other component passes though the first hologram-type polarized-light splitting element 32a and enters the liquid crystal display panel 31 in which the plane-polarized light is modulated. An unnecessary image-light component is further diffracted by the second hologram polarized-light splitting element 32b and then totally reflected. The liquid crystal projection display according to the present invention can be free from heat affection and aging because it uses the hologram-type polarized-light splitting element with no absorption of unnecessary light, instead of conventional dichroic type polarizing plate, as polarization elements before and after the liquid crystal display panel.

Fig. 8 is a schematic construction view showing an essential portion of a liquid crystal projection display according to another embodiment of the present invention. In Fig. 8, the liquid crystal display comprises a liquid crystal display panel 41, a hologram-type polarized-light splitting element 42, an incident-side polarizing plate 43a and an emergent-side polarizing plate 43b. In this embodiment, the liquid crystal display panel 41 is provided with the incident-side polarizing plate 43a disposed in the front thereof and the emergent-side polarizing plate 43b disposed in the rear thereof. Furthermore, a hologram-type polarized-light splitting element 42 is disposed between the liquid crystal display panel 41 and the emergent-side polarizing plate 43b. Natural light emitted from a light source (not shown) through an optical system (not shown) enters the incident-side polarizing plate 43a in which one of the two plane-polarized light components having different polarization directions perpendicular to each other is absorbed and the other component is allowed to pass therethrough and enter the liquid crystal display panel 41 in which the plane-polarized light is then modulated. An unnecessary polarized-light component is diffracted by the hologram-type polarized-light splitting element 42 and then completely reflected from the boundary, not reaching the emergent-side polarizing plate 43b that can be thus protected from being heated by absorption of the unnecessary light and, therefore, from heat-aging.

THE INDUSTRIAL APPLICABILITY OF THE INVENTION According to the present invention, it is possible to provide a hologram-type polarized-light splitting element that can accurately split polarized light into two plan-polarized components by means of a hologram having adapted diffraction properties and can operate without being heated due to absorption of light and can maintain high reliability of its performance for a long service life because it does not absorb unnecessary light components in distinction from conventional dichroicabsorption type polarizing elements. Furthermore, the element is a thin plate that can form a compact optical system because it can be disposed at right angles to the optical axis thereof.

According to the present invention, it is also possible to provide a hologram with preset conditions of diffraction properties necessary for splitting polarized-light at high degree of separation.

According to the present invention, it is further possible to provide a liquid crystal display having a compact optical system that can split incident polarized light into components without being heated because it does not absorb unnecessary light and does not require elongation of the back focal length of a projection lens.

P:\WPDOCS.\md\pcci\7514380.doc-14 Augus 2001 18- Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

0 *o ooo••*

Claims

14300.doc-14 August 2001 -19- The claims defining the present invention are as follows: 1. A hologram-type polarised-light splitting element including a glass substrate with a hologram formed thereon, which element is capable of splitting incident light into two plane-polarised components perpendicular to each other by allowing one of the components to pass the hologram after diffraction and the other component to pass therethrough without diffraction, and which element is capable of selecting and outputting only the non-diffracted plane-polarised component of the light in the specified direction and, at the same time, is capable of totally reflecting the diffracted plane- polarised component at least once at a boundary between the glass substrate and medium 10 surrounding the element.

2. A hologram-type polarised-light splitting element as defined in claim 1, wherein the hologram is given diffraction properties for obtaining diffracted plane-polarised light and non-diffracted plane-polarised light from a p-plane-polarised component and an s- 15 plane-polarised component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a splitting angle of degrees with the direction of the latter. o

3. A hologram-type polarised-light splitting element as defined in claim 1, wherein the hologram is given diffraction properties for obtaining diffracted plane-polarised light and non-diffracted plane-polarised light from a p-plane-polarised component and an s- plane-polarised component, respectively, of incident light at a high separation degree by diffracting the former component to travel in the direction making a separation angle of 48.2 degrees with the direction of the latter.

4. A liquid-crystal projection display including a liquid crystal panel and at least a first-polarising element and a second-polarising element on the incident side and an emergent side, respectively, of the liquid crystal panel, wherein the first and/or second- polarising element is the hologram-type polarised-light splitting element of claim 1 or 2 or 3. P:\WPDOCS\anmd\speci\75143KO.doc-I4 August 2001 20 A hologram-type polarised-light splitting element substantially as herein described with reference to the accompanying drawings. DATED this 13th day of August, 2001. SHARP KABUSHIKI KAISHA By Their Patent Attorneys DAVIES COLLISON CAVE I0